Massachusetts Institute of Technology
Engineering Systems Division
Working Paper Series
ESD-WP-2002-01
ESD Terms and Definitions (Version 12)
ESD Symposium Committee:
Tom Allen, Co-Chair
Don McGowan
Howard W. Johnson Professor of Management
Professor of Engineering Systems
Associate Director, Office of Corporate
Relations
Joel Moses, Co-Chair
Chris Magee
Institute Professor
Professor of Computer Science and
Engineering and Engineering Systems
Professor of the Practice of Engineering
Systems andMechanical Engineering
Dan Hastings
Professor of Civil and Environmental Engineering
and Engineering Systems
Professor of Aeronautics and Astronautics
and Engineering Systems
Fred Moavenzadeh
Debbie Nightingale
Seth Lloyd
Professor of the Practice of Aeronautics and
Astronautics and Engineering Systems
Associate Professor of Mechanical
Engineering and Engineering Systems
Dan Roos
John Little
Professor of Civil and Environmental
Engineering and Engineering Systems
Institute Professor
Dan Whitney
Senior Research Scientist, Center for
Technology, Policy and Industrial Development
October, 2001
DRAFT
ESD Terms and Definitions (Version 12)
The ESD Symposium Committee
October 19, 2001
I.
BASIC TERMS RELATED TO ENGINEERING SYSTEMS
Engineering – bringing to reality useful artifacts and algorithms that heretofore did not exist; in
English, especially in England, the term is often associated with maintenance and operation,
especially of engines, but the French root (ingénieur) is related to ingenious (ingénieux).
System – A set of interacting components having well-defined (although possibly poorly
understood) behavior or purpose; the concept is subjective in that what is a system to one person
may not appear to be a system to another
Engineering System – a system designed by humans having some purpose; large scale and
complex engineering systems, which are of most interest to the Engineering Systems Division,
will have a management or social dimension as well as a technical one.
System Environment – a set of conditions external to and affecting a system; environments
include both natural and man-made conditions
Natural environment – natural surroundings and context (e.g., air, water); the natural
environment can sometimes be considered the core of a system, with engineering systems
providing interfaces to it
Complex system – a system with components and interconnections, interactions, or
interdependencies that are difficult to describe, understand, predict, manage, design, or change.
The concept of complexity has several meanings: dynamic (in the sense of complex
behavior), and static or structural (in the sense that a complex system is composed of
many components interconnected in intricate ways). In addition, while the complexity of
a system can be a quantifiable and even an absolute quantity, what makes a system
appear complicated to people is subjective (simplicity is in the eye of the beholder); how
complicated the system appears depends on the nature of the interface of the system that
is presented to its users.
Interdependence – the relationship between entities that cannot exist or operate without
each other; independent entities exist and operate without influence from each other;
interdependencies may be intended or unintended
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Interaction – the property of entities that exchange something material or immaterial that
constitutes or contributes to their interdependence
Interconnection – the relationship between entities that are physically or abstractly
connected and the connection provides the pathway for the interaction; software
connections are often abstract
These three terms are clearly interrelated (pun intended). We distinguish at least three
types of interdependence: one is the interdependence that may occur among components
or subsystems in a given design of a system, a second is the interdependence created
when global constraints (such as weight, volume, cost, or 2nd Law of thermodynamics)
force a redesign, and a third is one that occurs as a result of subdivision of tasks or the
management of the flow of materiel or information.
Components – parts of a system relative to that system; a component can be a system too if it
contains other components
Large scale systems – systems that are large in scale and/or scope; such systems have a large
number of components; as a result large scale physical systems will be distributed over a region
that is large relative to its smallest components;
Designing –an open-ended human process whereby plans for useful artifacts and processes are
created
Function(s) –broad: desired behavior(s) of a system or a component; these behaviors are
presumably desired because they contribute to a stated purpose; more specifically, fundamental
behaviors (not including the ilities – see below) of an engineering system that fulfill its stated
purpose.
Requirements – the combined set of functions, ilities (see below), and constraints (e.g., weight,
volume, cost, physical laws) that an engineered system is supposed to achieve, deliver, or
exhibit; functions can be said to be what a system ‘does,’ whereas the ilities and constraints are
properties that a system ‘has’
Effectiveness –ratio of function(s) achieved to the totality of functions desired
Efficiency – ratio of function(s) achieved to resources used
Life cycle – The sequence of phases that an engineering system undergoes, which can be divided
into three major parts: Conceiving, developing, and deploying. Conceiving includes
identification of need/opportunity, initiation of requirement elicitation and gathering. Developing
includes analysis, design, implementation manufacture or production, and testing. Deployment
includes assimilation, use, maintenance, modification or upgrade, repair, retirement, dismantling,
recycling, disposal, erasure or remediation, and possible replacement; replacement is a critical
element in creating a cycle
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Systems changes – multiple dimensions by which systems change, including the rate of change
(e.g., evolutionary, moderate, revolutionary), the structural direction of change (e.g., top-down,
bottom-up, networked), or the breadth of change (e.g., pilot initiatives, wall-to-wall)
II.
ILITIES AND RELATED SYSTEM ISSUES
Ilities- requirements of systems, such as flexibility or maintainability, often ending in ‘ility;’
requirements of systems that are not necessarily part of the fundamental set of functions or
constraints
Flexibility – the property of a system that is capable of undergoing changes with relative ease.
Such changes can occur in several ways: a system of roads is flexible if it permits a driver to
reach from one point to another using several paths; flexibility may indicate the ease of
‘programming’ the system to achieve a variety of functions; flexibility may indicate the ease of
changing the system’s requirements with a relatively small increase in complexity (and rework)
Agility – ability of a system to be both flexible and undergo change rapidly
Robustness - demonstrated or promised ability to perform under a variety of circumstances;
ability to deliver desired functions in spite of changes in the environment, uses, or internal
variations that are either built-in or emergent (see below)
Fail-safe - ability to be guided to a safe state, if the system cannot deliver the full desired
function due to failure(s)
Adaptability – the ability of a system to change internally to fit changes in its environment. In
our definition above a flexible system is usually modified from outside the system. An adaptable
system may undergo self-modification (e.g., a thermostat controlling the heating of a subsystem)
Scalability- the ability of a system to maintain its performance and function, and retain all its
desired properties when its scale is increased greatly without having a corresponding increase in
the system’s complexity
An increase in scope (that is, an increase in the system’s functional capabilities) usually
involves an increase in scale, yet scalability does not normally imply ease with increasing
the scope of a system without unduly increasing its complexity. Such ease is usually
related to the structure of the system’s architecture (see below) and its flexibility
Modularity –the degree to which the components of a system can be designed, made, operated,
and changed independently of each other
Safety- the property of being free from accidents (see below) or unacceptable losses
Durability – ability to deliver a specified level of function for a specified length of time
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Sustainability – broad: maintaining economic growth and viability while meeting concerns for
environmental protection, quality of life, and social equity; narrow: a property of an engineering
system having optimal resource preservation and environmental management over time
Quality- ability to deliver requirements at a “high” level, as perceived by people relative to other
alternatives that deliver the same requirements
Reliability – the probability that a system or component will satisfy its requirements over a given
period of time and under given conditions
Repairability –the ability to be returned to the original state of function when some function is
lost
Maintainability - the ability of a system to be kept in an appropriate operating condition; the
system should also possess the property of repairability
III.
DESIGN/MANUFACTURING CONCEPTS AND APPROACHES
Manufacturing – The processes by which materials are made, parts or components are fabricated
from materials, and products are assembled from parts; software is implemented rather than
manufactured
Manufacturing Systems – The equipment, processes, people, organization and knowledge, as
well as the interactions of these that are involved in the manufacturing of a given end product
Lean Manufacturing – A pull based and flexible manufacturing system that is responsive to
customer demand, where skilled workers, just-in-time manufacturing processes, and continuous
improvement are combined to produce perfect first-time-quality output
Systems architecting - The process by which standards, rules, system structures and interfaces
are created in order to achieve the requirements of a system; trade-off studies may precede the
determination of system requirements
Systems engineering –a process for designing systems that begins with requirements, that uses
and/or modifies an architecture, accomplishes functional and/or physical decomposition, and
accounts for the achievement of the requirements by assigning them to entities and maintaining
oversight on the design and integration of these entities; systems engineering originally arose in
the context of aerospace projects in the 1950’s, but has been applied more broadly since then.
Systems architecting creates a system design at a high, abstract level, whereas systems
engineering is often associated with refining such a design; by blending the two processes
one accomplishes the assignment of functions to physical or abstract entities, and the
definition of interactions and interfaces between the entities
Optimization – a process or methodology for maximizing the function of a system
Multi-criteria optimization – the simultaneous optimization of several criteria
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Functional decomposition - the division of functions into sub-functions while retaining all inputs
to and outputs from the level above; the decomposed elements perform all the functions of their
parents in the decomposition
Logical decomposition – the division of information system components into their logical
constituent parts
Physical decomposition –the division of physical systems into simpler subsystems and
components
Integration –the act of anticipating or executing a combination of components of a system with
the expectation that all system requirements will be achieved.
System structures – abstractions useful within systems in order to understand, control and
facilitate the complex interactions – current possibilities include platforms, modules, networks,
teams, and hierarchies, and subsets and combinations thereof
Hierarchy – Hierarchies create a ranking of elements in a system. In a layered hierarchy one of
any pair of elements is above, below or at the same level as the other. In a tree-structured
hierarchy one adds the possibility that a pair of elements have no hierarchical relationship.
Reuse –In the design of systems, repeated use or application in different places of the design of
parts, manufacturing tools and processes, analysis, and particularly knowledge gained from
experience; using the same object in different systems or at different times in the same system
Module – A part of a system that is constructed to have minimal, standardized interactions with
the rest of the system (and is thus often reusable).
Platform –a module or set of components that splits a system into two parts so that changes can,
in principle, be made on either side of the platform interface without affecting the other side as
long as appropriate standards are followed; platform implementation: all parts or components on
the side of the platform interface farther from the end user, namely the parts or components
needed to achieve the desired abstract interface.
Integral architecture – an architecture in which the number of functions or behaviors is
significantly larger than the number of designed entities or components
Modular architecture – an architecture in which the number of functions is roughly comparable
to the number of designed entities or components; an architecture in which the interactions and
interfaces between the components are relatively simple
Interfaces – a boundary or interconnection between systems or their components that define or
support interrelationships; interfaces may be intended or unintended
IV.
RISK/UNCERTAINTY/SAFETY IN DESIGN/MANUFACTURING AND OPERATION
Ambiguity – open to having several possible meanings; may also be uncertain
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Uncertainty – related to being not clearly or precisely determined
Accident – an undesired and unplanned (but not necessarily unexpected) event that results in (at
least) a specified level of loss (called a loss event); losses can be economic losses, losses of
human lives, losses of function, losses of time, etc.
Hazard – a state or sets of conditions that, together with worst-case external conditions, can lead
to an accident
Risk – the level of hazard combined with the likelihood of the hazard leading to an accident, and
the duration or exposure of the hazard; a combination of the likelihood, severity and lack of
detectability of an accident or loss event
V.
MANAGEMENT AND RELATED SOCIAL SCIENCE ISSUES
Enterprise – a defined scope of economic organization or activity, which will return value to the
participants through their interaction and contribution
Lean enterprise – an enterprise that delivers value to its stakeholders, while minimizing waste
(waste can be in terms of materials, human lives, capital, time, physical plant, equipment,
information, and energy)
Learning Organization - an organization that systematically reviews its experience with its
internal and external environment and acquires knowledge in order to improve its functioning
Negotiation – an interactive process aimed at communication and agreement among multiple
stakeholders who have both common and competing interests
Socio-technical systems – broad: systems in which both human and non-human elements
interact, where the social or management dimensions tend to dominate
Socio-technical systems design – the design of work systems that attempts to optimize human
psychological and physiological dimensions along with the technical aspects
Policy studies – studies of courses of action, chosen from alternatives, that guide present and
future decisions by, for example, governmental bodies
Interdisciplinary – involving two or more academic disciplines or professional practices
VI.
GENERAL CONCEPTS AND APPROACHES RELATED TO SYSTEMS
System point of view – a conviction that system behaviors are qualitatively different from the
behaviors of a system’s components, that system design requires doing more than designing the
components, and that special effort is required to understand systems and their behavior over and
above what is required to understand any individual component
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System thinking – includes holism, an ability to think about the system as a whole; focus, an
ability to address the important system level issues; emergence (see below), recognition that
there are latent properties in systems; and trade-offs, judgment and balance, which enable one to
juggle all the various considerations and make a proper choice
System theories –theories that attempt to explain the interacting and combining behavior of a
system as well as how the interaction of its components contribute to the behavior of the system
System Modeling – vocabularies, symbols, rules, and representations (behavioral, structural) that
make use of the vocabularies, symbols, and rules for the purpose of displaying and predicting the
structure and behavior of systems, or which represent in symbolic form or operational ways one
or more aspects of the system under study
Emergent properties – properties or behaviors of a system that are discovered (i.e., properties
that were there but latent), those that emerge spontaneously over time or space, and those that
arise in response to behavior of other systems and environments; in a hierarchical view of
systems, emergent properties show up at one level of the hierarchy, but not at lower levels
Synergy –mutually advantageous behaviors or properties that exist only because distinct
elements are joined or can interact
VII.
SYSTEM THEORIES
General Systems Theory – Originated by Bartanlaffy in the 1950’s; an approach to modeling
systems using sets of differential equations
Cybernetics – Originated by Norbert Wiener; models systems using feedback processes
System Dynamics – Originated by Jay Forrester; used to model systems in the social sciences
and management; uses networks of nodes and feedback relationships amongst them
Complexity theory (in the sense of the Santa Fe Institute) – a set of approaches to understanding
systems that encompasses chaos theory and related theories; used to understand biological
systems as well as physics-based ones.
Complexity theory (as used in computer science) – approaches to the analysis of the computing
time and effort required by various algorithms; usually relies on techniques in combinatorics and
logic
Operations Research – a scientific approach to executive decision-making, including problem
formulation, mathematical modeling, and system optimization.